1. Field of the Invention
This invention concerns pulse oximeters, and more particularly devices and methods for testing or calibrating pulse oximeters.
2. General Discussion of the Background
Continuous assessment of arterial oxygenation is important in the treatment of critically ill or anesthesized patients. Such individuals are often dependent upon artificial life support systems which sufficiently oxygenate their blood to prevent permanent physical impairment, brain damage, or death. Blood oxygen concentration levels in such patients must be carefully maintained within a narrow range to prevent serious physical consequences. Premature infants, for example, must have a blood oxygen content below about 95% to prevent retinal damage and above about 90% to prevent respiratory distress.
Several methods have been devised for continuously monitoring blood oxygen levels. Many of these methods involve invasive removal of blood for analysis, which provides only intermittent information. Transcutaneous oxygen tension measurement provides continuous information but requires special site preparation, airtight probe mantling and a potentially harmful local heat source to induce arterialization. In addition to these serious drawbacks, transcutaneous oxygen monitoring often fails accurately to reflect true arterial oxygenation.
Pulse oximeters, such as those shown in U.S. Pat. No. 4,167,331, European patent application Publication No. 0 104 771, and European patent application Publication No. 0 102 816 are most convenient for monitoring blood oxygen concentrations. Such pulse oximeters function by positioning a pulsating arterial vascular bed between a two wavelength light source and a detector. The pulsating vascular bed, by expanding and relaxing, creates a change in the light path length that modifies the length detected and results in a plethysmograph waveform. The amplitude of the varying detected light depends on the size of the arterial pulse change, the wavelength of light used, and the oxygen saturation of the arterial hemoglobin. The detected pulsatile waveform is produced solely from arterial blood using the amplitude of each wavelength and Beer's law. An exact beat-to-beat continuous calculation of arterial hemoglobin oxygen saturation can thereby be obtained with no interference from surrounding venuous blood, skin, connective tissue, or bone.
A typical pulse oximeter includes a probe which is attached on either side of a distal digit, such as the tip of a finger. The probe includes the light source and the detector which are held in opposing relationship to one another on either side of the finger such that the light source directs a beam of light through the finger and towards the detector. Most pulse oximeters use simple detecting circuitry with diodes that have broad spectral sensitivity. The light source emits wavelengths of red and infrared light which correspond to absorption peaks of oxyhemoglobin and deoxyhemoglobin in red blood cells entering the capillaries during systoly. A background absorption occurs from the hemoglobin remaining in small vessels during diastoly and from general tissue absorption. By rapidly alternating the wavelengths of light transmitted through the tissue, the difference in absorption for total hemoglobin and oxyhemoglobin can be measured for each pulse of arterial capillary blood. An estimated percentage of oxygenated hemoglobin in each pulse can then be calculated from the difference in absorption.
Several pulse oximeters employing these principles are now available. The Nellcor pulse oximeter Model N-100 is available from Nellcor Inc. of Hayward, Calif. Other such devices include the Ohmeda Biox 3700 pulse oximeter and the Novametrix pulse oximeter.
Although pulse oximeters are valuable tools in continuously monitoring oxygenation levels of blood hemoglobin, there is presently no way for a user to test whether a pulse oximeter is accurate. A rotating blade of a graded neutral density filter material is proposed as a calibration device in European patent application Publication No. 0 102 816. But, making such a graded neutral density filter would require a major engineering effort. No such filter material is now known to exist. A careful and convenient method of calibrating pulse oximeters is needed since the oxygenation level of a patient's blood hemoglobin must be maintained within a narrow and often critical range. Slight deviations from the range must be accurately detected by the oximeter.
It is therefore an object of the present invention to provide a device and method for testing and calibrating pulse oximeters.
It is yet another object of the invention to provide such a device which is convenient and simple to use.
The foregoing and other objects and advantages of the invention will become more apparent from the following detailed description of the preferred embodiments which proceed with reference to the accompanying drawings.